Transonic Systems recently introduced novel ultrasound dilution method for routine access flow and access recirculation measurements that was successfully validated in animal experiments and clinical studies during the Phase I. We propose to: (1) Collaborate with key researchers to establish diagnostic significance and patient benefit by developing clinical strategies to monitor access device condition to optimize patency, including: (a) single- or combined screening criteria (access flow, access hydrodynamic resistance, intra-access pressure etc.) for access device condition; (b) develop thresholds for diagnostic parameters for elective minimally invasive interventions. (2) Increase the monitor's measurement sensitivity. (3) Simplify dilution methodology for screening use. 4) Improve the protocols and technologies so that, during Phase III, a stand-alone (no external computer) meter can be built and marketed which will allow nurses to take routine patient measurements of hemodialysis effectiveness and access shunt patency, without requiring knowledge of indicator dilution principles and knowledge of how "proper" indicator dilution curves are supposed to look like. PROPOSED COMMERCIAL APPLICATION: Access flow and recirculation measurement apparatus to be developed under this grant has the potential to become a routine measurement tool for hemodialysis, first as a stand- alone bedside monitor, later as a feature embedded in hemodialysis machines. If successful, this technology will improve hemodialysis patient management, reduce patient management costs and improve patient quality life. Once the procedure becomes a reimbursable expense, its market consist of every hemodialysis treatment center.